This invention relates to a synchronizing signal detector for detecting a received television synchronizing signal and, more particularly, to such a synchronizing signal detector which avoids false detection and, moreover, is readily adapted for use in an AFT circuit.
In a television receiver, and particularly, a color television receiver, it is desirable that the tuning section be tuned as closely as possible to the picture frequency of a broadcasted television channel. It is well known to provide automatic fine tuning (AFT) circuitry for effecting an automatic fine tuning operation in order to bring the tuning condition of the television receiver as closely as possible to the picture frequency of a broadcasted television channel. A conventional AFT circuit relies upon the intermediate frequency (IF) signal and, particularly, the level of the IF carrier, as an indication of whether the tuning section exhibits its desired tuning condition.
A typical composite color television signal includes a so-called picture carrier of frequency f.sub.p, a sound carrier of frequency f.sub.s and a chrominance carrier of frequency f.sub.c. For a given television channel, the picture carrier and sound carrier frequencies are separated by 4.5 MHz, with the sound carrier frequency being the higher frequency. If the IF signal is used as an AFT control signal, it is possible that a received sound carrier f.sub.s may, erroneously, be interpreted as a picture carrier f.sub.p, with the result that the tuning section is tuned, erroneously, to the sound carrier and not to the picture carrier. It is a general problem that, when the IF signal is used as an AFT control signal, the automatic fine tuning operation may be controlled in response to the received picture carrier f.sub.p, the received sound carrier f.sub.s or the received chrominance carrier f.sub.c. Consequently, an erroneous automatic fine tuning operation may be carried out, and the tuning section of the television receiver may be tuned to an improper frequency such that the program information which is broadcasted over a television channel is not received properly.
To avoid this problem based upon using the IF signal as an AFT control signal, it has been proposed to rely upon the horizontal synchronizing signal included in the received composite television signal to control the AFT operation. The basis for this proposal is that if the tuning section of the television receiver is tuned properly to a picture carrier f.sub.p, then the horizontal synchronizing signal will be detected easily, and this detected horizontal synchronizing signal will be of at least a predetermined level. If the television receiver is tuned improperly, the horizontal synchronizing signal will not be received and, consequently, separated or detected.
An implementation of the foregoing proposal to control an AFT operation with the separated horizontal synchronizing signal includes a synchronizing signal separator, a low pass filter and a peak detector. The horizontal synchronizing signal is separated from the received composite television signal, filtered and peak-detected. If the resultant DC level produced by the peak detector is less than a predetermined level, it is assumed that the tuning section is not tuned properly to a picture carrier f.sub.p. The peak-detected DC signal then is used to adjust the tuning condition until the peak-detected DC signal exceeds a predetermined threshold. At that time, it is assumed that the tuning section is tuned properly to a picture carrier f.sub.p.
However, in the foregoing implementation, it is possible that the output of the synchronizing signal separator is not a horizontal synchronizing signal but, rather, is a sound signal having an amplitude component that is interpreted by the synchronizing signal separator as being the synchronizing signal, this sound signal being transmitted by the sound carrier f'.sub.s of an adjacent television channel. With this misinterpretation, the tuning section of the television receiver may be tuned erroneously to the adjacent-channel sound carrier f'.sub.s. Furthermore, because of cross modulation distortion attributed to high amplitudes of the picture carrier f.sub.p and the sound carrier f.sub.s in a given television channel, a false, or spurious, carrier F.sub.IM may be detected. This false carrier f.sub.IM may result in the erroneous detection of a false synchronizing signal, which has the effect of controlling the AFT operation such that the tuning section is tuned to this false carrier f.sub.IM.